Image signal enhancement system for a scanning electron microscope

ABSTRACT

The invention pertains to a method and apparatus for reducing the effect of aberrations in an optical or electron-optical system. This is done by varying the focal point over a range and altering the reproduced images to produce a composite image substantially free of the said aberrations.

O United States Patent [191 [111 3,829,691 Hufnagel Aug. 13, 1974 IMAGE SIGNAL ENHANCEMENT SYSTEM [56] References Cited FOR A SCANNING ELECTRON UNITED STATES PATENTS MICROSCOPE 2,348,03l 5/1944 Rajchman 250/495 75 Inventor; R b E H f I Rid fi ld 2,464,396 3/1949 Hillier et al 250/495 Conn. Primary ExaminerWilliam F. Lindquist [73] Assignee: '{1lc1;v::ll](klI(l:-r l: Corporauon, Attorney, Agent, or FirmFrank J. Thompson [22] Filed: Sept. 22, 1969 [57] ABSTRACT 2 Appl. 59 7 The invention pertains to a n ethqd and apparatus for reducing the effect of aberratlons in an optical or electron-optical system. This is done by varying the focal [52] U.S. Cl. 250/311, 250/307 point Over a range and altering the reproduced images [51] Int. Cl H0l 37/26, GOln 23/00 to produce a composite image Substantially free of the [58] Field of Search 350/495 A, 493.20?2,; 425 Said aberrations 1 Claim, 6 Drawing Figures l Foals swap ,l 12

cz/vmn TOR J4 I l I KL/jg 2e 15 I l 24 l K 55 7 04 I -L- H T SCfi/VA/M/G I I l I I r GE/WERHTOR I I T V \l X I 25 L J IMHGE S/GlY/IL PROCESSOR 15 IMAGE SIGNAL ENHANCEMENT SYSTEM FOR A SCANNING ELECTRON MICROSCOPE This invention relates to imaging apparatus such as microscopes, telescopes and the like. The invention relates more particularly to a method and apparatus for reducing the effect of aberrations in a reproduced image.

In an imaging apparatus of the type to which this invention applies, light rays, electrons or other particles are caused to converge at a focal plane by optical or electro-magnetic focusing means. The focal plane may represent for example the image plane of a distant object in an optical system or the surface of a material which is to be bombarded by focused electrons of an electron microscope. In these and other imaging systems the focusing means of the apparatus creates symmetrical aberrations which prevent the apparatus from exactly focusing the light rays or electrons in the desired manner. These aberrations can be inherent in the apparatus or can result from limitations which are introduced and deemed tolerable in view of design cost or production cost factors.

A scanning electron microscope is a particular example of a system in which these aberrations occur and to which this invention is directed. The scanning electron microscope is an imaging system wherein an energized beam of electrons is scanned in mutually perpendicular directions over a body of material and an electronic output signal is thereby generated. The microscope includes an electron-optical arrangement which focuses the beam at the surface of the sample material. Because of different energies of the accelerated electrons and other factors related to the design of the focusing arrangement, the microscope exhibits undesirable spherical aberrations which reduce the conformity between the sample object and an image reproduced from the electronic signal.

It is an object of the present invention to provide a method and apparatus for improving conformity between a reproduced image and its object in an imaging system.

Another object of the invention is to provide a method and apparatus for reducing the disadvantageous effect on image reproductions caused by symmetrical aberrations existing in imaging systems.

A more particular object of the invention is to provide a method and apparatus for increasing conformity between a reproduced image and its object in a scanning electron microscope.

A further object of the invention is to provide a signal enhancing method and apparatus for enhancing the image information content for an electrical image signal represented in serial form by a scanning electron microscope.

In accordance with the general features of the present invention a method for increasing conformity between a reproduced image and an object in an imaging system comprises the alteration of a focusing parameter of the system and the corresponding modification of reproduced images or image signals in accordance with a predetermined correction function.

Apparatus in accordance with a feature of the invention includes means for modifying a focal parameter of the imaging system and means for altering reproduced images associated with the altered focal parameter and providing a composite image representative of an enhanced focal characteristic of the system.

These and other objects and features of the invention will become apparent with reference to the following specifications and the drawings, wherein:

FIG. 1 is a diagram in block form illustrating an electron scanning microscope form of imaging system employing one embodiment of the present invention;

FIG. 2 is a more detailed block diagram of the components of a signal processor of the imaging system of FIG. 1; and,

FIGS. 3a, 3b, 3c and 3d are diagrams useful in explaining aberration defects in imaging systems.

The defects in an imaging system to which the present invention is directed comprise symmetrical aberrations which as is known are principally dependent on a focusing characteristic of the system. In a purely otpical imaging system the aberration can result from a defect in a lens for example, while in an electronic imaging system such as the electron microscope, the defect may exist in the focusing arrangement. The aberration may be represented by an infinite number of focal points each of which exhibits a particular optical transfer function. Exemplary optical transfer function at two focal points are illustrated in FIGS. 3b and 3c. The ordinate Q in these diagrams represents the unitless optical transfer function having a value equal to the ratio of the amplitudes of a signal before and after transmission. The abscissa (f) comprises spatial frequency which is measured in cycles per unit of length. It is desirable in imaging systems having symmetrical aberrations to provide a transfer function having a value of unity. The chart of this particular function is represented by FIG. 3a.

In accordance with a feature of this invention the focusing parameter of the apparatus is altered to provide a plurality of focal locations each having an associated optical transfer characteristic. The image produced at each of these locations is altered in order to establish a resultant composite artificial image which artificial image is the image which would be provided by an imaging system free of the aberration or having an aberration of reduced effect. An imaging system providing the artificial image would have a transfer function Q substantially closer to unity within the spatial frequency range of interest than that of the uncorrected system. The aberration limited imaging system thereby provides an image which would be provided by the apparatus if the aberration were reduced or non-existent.

This result is achieved in an electron scanning microscope for example wherein an output image signal is generated in serial form by varying the focal potential of the apparatus in order to vary the focal point and by multiplying the output signal by a suitable weighting factor which causes the transfer function of the system to approach the desired value of unity. In an optical system wherein the image at a particular focal location is photographically stored, the overall image enhancement may be accomplished by photographically storing the image for each focal location and by combining the stored images and multiplying each of the images by a weighting factor such as a variation in brightness or contrast in order to provide a resultant optical transfer function for the system more closely approaching unity than the transfer function of the original system.

More generally, if the transfer functions of FIGS. 3b and 3c represent the transfer function for definitive focal positions Z and Z respectively of an imaging system, and I and I represent the respective image or image signal of these locations, then there are weighting factors W and W such that W 1 W 1 1 where 1 represents an artificial image provided by an imaging system having an optical transfer function more continuously approaching unity over the spatial frequency range of interest than the transfer function of the uncorrected imaging system. This is expressed generally and approximately as:

1... W gf W(Z)I(Z)dZ 1) In these relationships, T: at a spatial frequency (f) is selected to be a desired value. The weighting factors W may be determined by the solution of these linear simultaneous equations wherein the spatial frequencies f f and f are selected for desired frequencies within a range of interest and the factors T T and T are known from a knowledge of the characteristic defect of the imaging system and its transfer function. The transfer function T= 2T T where W l and W +2 is plotted in FIG. 3d as are the transfer functions (T which is derived from the transfer function of FIG. 3b

and the transfer function (2T which is derived from the transfer function of FIG. 30. Thus, in accordance with features of the invention a focal parameter is varied in steps or continuously and the image signal, be it an optical or electrical signal, which is transmitted in parallel or in serial fashion is modified by the weighting function associated with each focal location. The weighted image signals are combined to provide a composite artificial image signal I: having a transfer function more closely approaching the desired flat transfer function.

The features of the present invention will now be explained with reference to a particular imaging system illustrated in FIG. 1. The imaging system of FIG. 1 comprises a scanning electron microscope wherein the microscope is indicated generally by the dashed rectangle 10. The microscope includes a source of electrons 12 which comprises a heated cathode or other known source and means for accelerating and focusing an electron beam 14 at a target surface 16 which positions a sample under analysis in the path of the impinging electron beam. The beam forming and accelerating electrodes are not illustrated in detail, and the focal means is shown to comprise electrostatic focal plates 18, although magnetic focal means may alternatively be employed. A d.c. potential is provided by a battery source 20 and is coupled to the focal plates 18. The source of d.c. potential is illustrated as a manually adjustable source. The electron beam is periodically deflected in a scannng raster 22 across the sample by means comprising electrostatic scanning plates 24 to which is applied a saw-toothed scanning voltage which is derived from a scanning generator 26. Scanning along only one axis is illustrated. Electrons which impinge upon the sample under analysis will generate a signal such as by secondary emissions and these secondary electrons impinge upon a detector 28 to which is coupled a video amplifier 30. The amplified image signal derived from detector 28 is then applied through an image signal processor 32, which is described in greater detail hereinafter, to a control electrode of a cathode ray tube 34 for display of the image. A scanning electron beam in the cathode ray tube is synchronized with the scanning electron beam of the scanning electron microscope 10 by coupling the deflection waveform from the scanning generator 26 to deflection plates 36 of the cathode ray tube.

The scanning electron beam 14 is deflected in mutually perpendicular directions such as the x and y direction over information elements 38 of the sample. The video signal therefore presents intelligence in serial form and this intelligence is representative of the sample at each information element. In an electron microscope of the nonscanning type, the beam may impinge upon a plurality of informational elements and the intelligence derived from these informational elements may alternatively be read out in parallel and stored for processing.

The microscope 10 will exhibit spherical aberrations which cause the image reproduced on the cathode ray tube screen to differ in conformity from the object comprising the sample under analysis. In accordance with a feature of this invention, the focus potential applied to the electrostatic plates 18 is periodically varied in a manner for causing a focus scan in the z direction at each informational element. Thus while the electron beam is scanning a raster in mutually perpendicular directions, a scanning focal voltage of greater frequency is derived from a focus sweep generator 40 and is coupled to the plates 18 for varying the focal point of electron beam 14 over a range of locations at each information element. In synchronism with this focal scanning, the signal output from the electron microscope is altered in a manner for enhancing the image produced. The image signal processor 32 will operate on the image signal to perform the function represented by equation 1, and the imaging signal I: is therefore an artificial image signal of the type which would be provided by this scanning electron microscope if the spherical aberrations were substantially reduced or were nonexistent.

As indicatedhereinbefore, a signal is altered by multiplying the signal by a weighting function which will vary the signal in a manner for enhancing the information content. FIG. 2 illustrates a particular arrangement of the signal processor 32 of FIG. 1 which will operate on the output signal of the video amplifier 30 to provide the enhanced signal 1:. The image signal processor of FIG. 2 includes a waveform generator .42 which generates a weighting waveform 44, which when multiplied by the image signal will provide the enhanced output signal 1:. The generator 42 comprises for example a digital shift register and associated output logic elements for establishing a desired correction waveform 44. Multiplication is effected in an operational amplifier by the application of the weighting voltage 44 and the video signal amplifier voltage to the inputs of an operational amplifier 46. Positive going components of the waveform 44 will be applied to the operational amplifier along one input line 48, while negative going components of the waveform 44 will be applied along an input line 50. The magnitude of the multiplicand is determined by the duration of the associated segment of the waveform 44. For example, t and 1 represent separate weighting factors for each focal location. The weighting factor and the duration of these segments is determined as indicated hereinbefore by the solution of the simultaneous linear equations. The output signal l= of the operational amplifier 46 comprises an enhanced image signal which would have been provided by the electron microscope if the aberration present in the apparatus were greatly reduced or nonexistent.

Although I have particularly described an apparatus and method for operating onjan image signal presenting intelligence in serial fashion as found in a scanning electron microscope, the invention described and claimed herein is equally applicable to optical as well as electrical arrangements and wherein the intelligence is presented in parallel as well as in serial fashion. In the case of an output image signal providing intelligence in parallel fashion, the information can be initially stored and then read out sequentially, at which time it may be multiplied by the weighting function. Similarly and as indicated hereinbefore, photographic or photoelectric output techniques may be employed with purely optical systems and the weighting function may be accomplished by varying brightness and contrast.

A method and apparatus has thus been described for enhancing an image produced in an imaging system having aberrations which ordinarily detract from the conformity to the image and the object.

While I have illustrated and described a particular embodiment of my invention, it will be understood that various modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. in a scanning electron microscope having a source of electrons, means for focusing the electrons at a sample object, means for scanning the electrons in mutually perpendicular directions across said object, and means for providing an electrical output signal representative of the object, means to reproduce an image of said object, said microscope exhibiting a symmetrical aberration, the improvement for increasing the conformity between a reproduced image and the object comprising means for varying the location of the focal point during a scanning interval at each of a plurality of information element locations and means for altering the output signal in accordance with a predetermined electrical signal. 

1. In a scanning electron microscope having a source of electrons, means for focusing the electrons at a sample object, means for scanning the electrons in mutually perpendicular directions across said object, and means for providing an electrical output signal representative of the object, means to reproduce an image of said object, said microscope exhibiting a symmetrical aberration, the improvement for increasing the conformity between a reproduced image and the object comprising means for varying the location of the focal point during a scanning interval at each of a plurality of information element locations and means for altering the output signal in accordance with a predetermined electrical signal. 